Maytansinoids are highly cytotoxic drugs. Maytansine was first isolated by Kupchan et al. from the east African shrub Maytenus serrata and shown to be 100 to 1000 fold more cytotoxic than conventional cancer chemotherapeutic agents like methotrexate, daunorubicin, and vincristine (U.S. Pat. No. 3,896,111). Subsequently it was discovered that some microbes also produce maytansinoids, such as maytansinol and C-3 esters of maytansinol (U.S. Pat. No. 4,151,042). Synthetic C-3 esters of maytansinol and analogues of maytansinol have also been reported (Kupchan et al., 21 J. Med. Chem. 31-37 (1978); Higashide et al., 270 Nature 721-722 (1977); Kawai et al., 32 Chem. Pharm. Bull. 3441-3451 (1984)). Examples of analogues of maytansinol from which C-3 esters have been prepared include maytansinol with modifications on the aromatic ring (e.g. dechloro) or at the C-9, C-14 (e.g. hydroxylated methyl group), C-15, C-18, C-20 and C-4,5.
The naturally occurring and synthetic C-3 esters can be classified into two groups:                (a) C-3 esters with simple carboxylic acids (U.S. Pat. Nos. 4,248,870; 4,265,814; 4,308,268; 4,308,269; 4,309,428; 4,317,821; 4,322,348; and 4,331,598), and        (b) C-3 esters with derivatives of N-methyl-L-alanine (U.S. Pat. Nos. 4,137,230; 4,260,608; 5,208,020; 5,416,064; and 12 Chem. Pharm. Bull. 3441 (1984)).        
Esters of group (b) were found to be much more cytotoxic than esters of group (a). Because maytansinoids are highly cytotoxic, they were expected to be of use in the treatment of many diseases, such as cancer. This expectation has yet to be realized. Clinical trials with maytansine were not favorable due to a number of side effects (Issel et al., 5 Can. Trtmnt. Rev. 199-207 (1978)). Adverse effects to the central nervous system and gastrointestinal symptoms were responsible for some patients refusing further therapy (Issel at 204), and it appeared that maytansine was associated with peripheral neuropathy that might be cumulative (Issel at 207).
However, forms of maytansinoids that are highly cytotoxic, yet can still effectively be used in the treatment of many diseases, have been described (U.S. Pat. Nos. 5,208,020 and 5,416,064; Chari et al., 52 Cancer Res. 127-131 (1992); Liu et al., 93 Proc. Natl. Acad. Sci. 8618-8623 (1996)).
U.S. Pat. Nos. 5,208,020, 5,416,064 and 6,333,410 disclose that a thiol-containing maytansinoid may be produced by first converting a maytansinoid bearing an ester group into maytansinol, then esterifying the resulting maytansinol with a disulfide-containing acyl N-methyl-L-alanine to yield disulfide-containing maytansinoids. Reduction of the disulfide group with dithiothreitol gave the thiol-containing maytansinoids. However, this process involves several inefficient steps that are cumbersome and result in only moderate yields.
More specifically, maytansinol is first derived from maytansine or other esters of maytansinol by reductive cleavage, such as with lithium aluminum hydride. (Kupchan, S. M. et al., 21 J. Med. Chem. 31-37 (1978); U.S. Pat. No. 4,360,462). It is also possible to isolate maytansinol from the microorganism Nocardia (see Higashide et al., U.S. Pat. No. 4,151,042). In one specific example, the conversion of Ansamitocin P-3 into maytansinol by reductive hydrolysis with lithium aluminum hydride in tetrahydrofuran at −5° C. is described (U.S. Pat. No. 4,162,940).
The next step in the process is the conversion of maytansinol to different ester derivatives using N-methyl-L-alanine derivatives, and suitable agents such as dicyclohexylcarbodiimide (DCC) and catalytic amounts of zinc chloride (see U.S. Pat. Nos. 4,137,230, 4,260,609, 5,208,020, 5,416,064 and 6,333,410; Kawai et al., 32 Chem. Pharm. Bull. 3441-3951 (1984)). In all cases, two diastereomeric products containing the D and L-aminoacyl side chains result, as does a small portion of unreacted maytansinol. In the processes previously described (Kupchan, S. M., 21 J. Med. Chem. 31-37 (1978); U.S. Pat. No. 4,360,462; U.S. Pat. No. 6,333,410), the desired L-aminoacyl ester is obtained after purification over two silica gel columns or a combination of silica gel columns and HPLC columns. In addition, because of complete racemization, the isolated yield of the desired L-aminoacyl isomer is only around 30%. Hence, the processes described thus far are cumbersome, uneconomical and poorly amenable to use on an industrial scale.
Accordingly, an improved process for the preparation and purification of thiol-containing maytansinoids, which predominantly results in the synthesis of the desired diastereomers, is greatly needed.